Thanks for sharing this, it shows great insight into the way we are, so genuinely identify with you. Lying when it is easier, or just as easy to tell the truth, I very much identify with, and it can be hard at times with this habit (or by now virtually default setting) to find what the truth is in a situation, particularly an emotionally charged one. I have found that when I bite too quickly, and agree with the version being put in front of me, I can sometimes live to regret it. What I need to do in highly charged situations, is leave space for myself to think about what someone has said, and ponder it for validity. There is anyway always the opportunity to do this.
So, there's a however coming here, did you see it?! I don't think you're accountable to your boyfriend in 'owning up' about every facet of your past. Your past, unless it has a direct bearing on your ability to be present in the relationship emotionally, physically, spiritually if that counts for you, has nothing to do with him, and is for you to share and reveal as trust develops, and intimacy grows. It's none of his business, although he can of course ask, you under on obligation to tell anyone absolutely everything. Wanting someone to reveal facts or history about themselves when they are patently not ready to do so, or are perhaps clearly uncomfortable, can be a form of manipulation, and can also be used later on, either way (y'know the spiel...anything you say can later be used in evidence against you...blah blah to project discomfort or unease. These are also the situations where 'omission' happens. Is that dishonesty then? Or is it a legitimate response to feeling uncomfortable about what we are being asked, because we intuitively know the question is a spiked one?
These are of course only my opinions, based on my experience. But the kind of questions that try and hit the jackpot of discomfort is key in relationships that are less than healthy. I'm not saying yours was, but only that latching onto this now seems to me to be convenient, and to be honest, not entirely frank, and lacking integrity on his part. You are absolutely right of course that his feelings are as they are, but it doesn't necessarily have to be that you agree with them. Part of the deal is we get to decide for ourselves what is true or not for us, even if that means disagreeing with someone, and (yikes!) standing up for ourselves. None of that has to involve pointing the finger either.
So, love too that you mention your side of the street (now I've written an entire post taking your ex's inventory!) but your post prompted a 'hang on, note of caution' in me. By no means makes me right though! As you say, clearing our side really is all we can do, and it sounds like you're making great progress, but do remember it is your 50%....no more, no less.
Wish you well